Archimedes' Principle of watermelons states that any watermelon juice which is absorbed by a marshmallow will make the marshmallow heavier and wetter and better tasting.

Modernly made famous by Tom Willett in his tutorial on how to eat a Watermelon.

Bob:"Can you explain Archimedes' Principle of Watermelons to me, Tom Willett?"
Tom:"When you combine a marshmallow and watermelon, and you spoon it around in the juices of watermelon and then you take it out you will notice that the marshmallow has taken on some of the properties of the watermelon; it is more pinkish-redish in appearance, and wetter."
Bob:"Thanks! I finally understand it."